AN INK-JET ARRAY

Abstract: An ink-jet array for a printer, comprising a number of elements, each element comprising an ink chamber (50) and means (45) for providing it with ink, and a piezo-actuator (43) rigidly secured to the ink chamber (50) on one side, the ink chamber being adapted so as to be brought into motion in response to an image signal that an ink droplet is ejected via a nozzle (49), and the ink chambers (50) of the array each being adapted to be brought into motion separately.

Description: AN INK-JET ARRAY The invention relates to an inkjet head array for a printer. An individual inkjet head of this kind is known from US-A-3 857 049, which describes an inkjet head made up of a tubular ink chamber provided with a nozzle, a piezo-element being disposed around the tube. The surface tension in the nozzle prevents the ink from escaping from the ink chamber if the element is not energised. An electrical pulse having a short rise time produces a sudden change of volume in the chamber, an acoustic pressure pulse of sufficient amplitude forming to overcome the surface tension. In this way an ink droplet is ejected. If a receiving sheet is now moved past the nozzle and the piezo-element is energised image-wise, an image of ink droplets will be formed on the receiving sheet. This principle is known as "drop-on-demand". US-4 546 361 discloses another inkjet head in which a single capillary tube is connected to one end of a tubular piezo-element which is disposed concentrically around said tube and the other side of which is connected, for example, to a fixed part of a printer. When the piezo-element is energised, the capillary tube moves axially so that an ink droplet is ejected via a nozzle provided in the capillary tube. A construction of this kind having a concentrically disposed piezo-element is not very suitable for integration in an inkjet array requiring a high nozzle density. The object of the invention is to construct a "drop-on-demand" system in array form so that a smaller head can be formed which has greater reliability and improved energy efficiency and which can eject droplets at a high frequency, so that it is possible to obtain a high-speed printer with high resolution. In an inkjet array according to the preamble of claim 1, to this end, according to the invention, the ink chambers (50) lie in a first plane, separated from each other and can each be brought into motion separately. With the inkjet array according to the invention, the formation of air or vapour bubbles in the ink chamber is reduced in comparison with the known systems, thus increasing the reliability of operation. The acceleration that each ink chamber receives is transmitted to the ink, which thus also experiences an acceleration, so that pressure waves are generated in the ink chamber, so that an ink droplet is ejected. This gives greater freedom in the design of an integrated inkjet array. The inkjet heads according to the invention are accordingly very suitable for forming a complete row of ink chambers close together. These and other advantages of an inkjet array according to the invention will be described hereinafter with reference to the accompanying drawings wherein: Fig. 1 diagrammatically illustrates a single inkjet head. Figs. 2 and 3 diagrammatically illustrate other embodiments of an inkjet head. Figs. 4a and 4b show an inkjet head array according to the invention. Fig. 5 is an illustration of another embodiment of an inkjet array. Fig. 6 is a top plan view of Fig. 5. Fig. 7 is a front elevation of Fig. 5 and Fig. 8 shows a single element of the array according to Fig. 5. Fig. 1 diagrammatically illustrates the principle of an inkjet head as used in an array according to the invention. An ink chamber 10 in the form of a glass capillary has a constriction at the top, thus forming a nozzle 11. The ink chamber 10 is stuck to a piezo-actuator or piezo-element 12. The latter is connected by one side to a fixed part 14, for example of a printer, while the ink chamber 10 can move freely with respect to said fixed part. This free movement can also be obtained by connecting the ink chamber 10 to the surroundings elastically, e.g. by a silicone resin or rubber. An ink supply chamber 13 is formed in the fixed part 14 and the ink chamber 10 leads into it. This ink chamber is completely filled with ink by the capillary action. Of course it is possible to provide the ink chamber 10 with ink in some other way or construct the ink supply chamber 13 in some other way. The piezo-element is also provided with connecting electrodes (not shown), by means of which a sinusoidal or pulsed voltage can be applied across the element 12. This element thus vibrates and this oscillation is transmitted to the ink chamber 10, which can thus perform a movement in the direction of arrow 16, since the d3, mode (length mode) of the piezo-element is mainly used. The required voltage across the piezo-element is typically 1 to 50 volts. This voltage is dependent on the thickness of the piezo-element, its volume, the rigidity of the connection between the piezo-element and the ink chamber, the dimensions of the ink chamber, and also physical properties of the ink and the droplets. As a result of the acceleration of the ink chamber acoustic pressure waves will be generated in this chamber and are propagated therein at the speed of sound. The speed of sound in ink depends, in the present configuration, inter alia on the ink properties and the ink volume. A characteristic measurement of the deflection of the ink chamber is 5 - 50 nanometres, and 0.1 - 2 bar for the amplitude of the pressure waves. By correct coupling of the acoustic impedance of the nozzle and the ink chamber, the acoustic waves can bring into motion in the nozzle the liquid which can be regarded as incompressible, and it is possible to achieve speeds of flow better than 10 m/s. The ink flowing from the nozzle is then formed into a droplet by the action of the surface forces. By correctly adjusting the piezo-element control, particularly as regards the pulse width, it is possible to generate in the ink chamber pressure waves which by interference yield a high amplitude and thus a high droplet speed for a relatively low control voltage. Correct breaking off of the droplet which is ejected can also be ensured by way of the movement of the ink chamber. A further advantage can be obtained in this way in respect of the final speed of the droplet, and in the prevention of small satellite droplets which have an adverse effect on print quality. In the case of a capillary 10 having a diameter of 100 ,em and a cross-section for the nozzle 11 of about 20 lim, ink droplets from 20 to 50 Rm are thus obtained. When the entire system resonates, a droplet frequency of about 500 kHz is obtained. By energising the piezo-element with one pulse, just one droplet is ejected. By supplying these energisation pulses to the piezo-element 12 in accordance with an image signal, while a receiving sheet 17 is fed along the nozzle in synchronisation with said image signal, an image formed by ink droplets can be obtained on said receiving material. In this mode about 50 kHz is obtained. Good results have been obtained with ink chambers having a diameter less than 0.2 mm and having a rectangular cross-section smaller than 0.04 mm2. Given an ink chamber cross-section of 0.2 mm, the diameter of the nozzle was 0.05 mm. Typical dimensions for the length of the chamber are a few millimetres. The choice of ink chamber length does not appear to be critical for a good drop-on-demand effect. The length of the ink chamber does determine the fluid resonance frequency. If the ink chamber behaves as an oscillatory cavity (this depends on the acoustic impedance of the nozzle and the ink supply opening), the higher natural frequencies in the liquid are equal to 80 x n kHz (for a speed of sound of 1000 m/s and an ink chamber length of about 6 mm). Other natural oscillations in the system may possibly also couple with the natural oscillations in the liquid. In practice it has been found that many of these natural oscillations can be damped by a choice of suitable material properties and geometries. Chamber lengths between 1 mm and 10 mm can be used. By using ink chambers 10 having a diameter of 120 lim, the thickness of the piezo-elements 12 being about 100 Rm, it has been possible to make an inkjet head with a straight row of nozzles having a total density of eight elements per mm. The ink supply chamber 13 can be common to all these ink chambers. The glass ink chambers 10 are secured to the piezo-elements 12 by means of a glue (Araldite AV 138, to which approximately 30% aluminium oxide was added). The rigidity of this connection appears to be very important for efficiency. With optimum rigidity it was found that 1 volt was sufficient to generate droplets. It is also possible to connect the ink chambers to the piezo-elements in some other way, e.g. bonding, welding or soldering etc. The transition between the ink chamber 10 and the nozzle 11 also has some influence on the range of action of the inkjet head, but in practice it has been found that both a gradual and an abrupt transition are satisfactory. Figs. 2, 3a and 3b show three other inkjet heads diagrammatically, using the same references as in Fig. 1 for like elements. In Fig. 2, use is mainly made of the d33 mode (thickness mode) of the piezo-element 12 by the choice and connection thereof, so that the capillary moves mainly in the axial direction of arrow 16. In Fig. 3, the ink chamber 10 is flexibly connected to the ink supply chamber 13 by means of a silicone rubber packing 19. Here the piezo-element 12 is used in the shear-stress mode, so that the capillary moves mainly axially. In the examples according to Figs. 1, 2 and 3, the ink chamber 10 is always moved substantially axially, perpendicularly to the receiving sheet 17. Figs. 4a and 4b show an inkjet head array according to the invention, Fig. 4b being a cross-section on x-x in Fig. 4a. A number of teeth 22, 23 are formed as a comb structure in a sheet of piezomaterial 20, 21. The piezo-material 20, 21 is provided with an electrode layer on both sides, such layer being removed in areas 36 in order to obtain elements which can be energised separately per tooth 22, 23. The electrode layers are provided with connecting electrodes 34, 35 for each element. Ink chambers 24, 25 are rigidly secured to the ends of teeth 22, 23. The ink chambers 24, 25 are made from silicon rods, in which chambers 30, 31 are etched on one side and lead into nozzles 32, 33. These ink chambers 24, 25 are closed by Pyrex plates 28, 29. The piezo-sheet 20, 21 is secured to a support 27 in which an ink supply chamber 26 is formed and is closed with silicone rubber 19. The ink chambers 24, 25 can be brought into motion independently of one another by energisation via connecting electrodes 34, 35. To produce an inkjet array of this kind, a sheet of piezo-electric material 20, 21 is used, a silicon strip being glued to one side and having a large number of chambers 30, 31 with nozzles 32, 33 etched therein; These chambers are then closed with a strip of Pyrex glass. Areas are removed from the plate by means of a diamond saw or by photolithographic techniques, to form teeth 22, 23 with the separate ink chambers 24, 25 connected thereto. The electrode layer is also removed from the sheet in areas 36 by means of mechanical or photolithographic techniques and connecting electrodes 34, 35 are applied. An inkjet array of this kind can be made singly or, as described above, in a double construction over the full width of a receiving sheet for printing, and also in the form of a number of smaller modules which are provided in a printer in known manner stepwise or contiguously. It is also possible to move a smaller module width-wise over a receiving sheet, to give a line print In the case of a double row of inkjet heads, th E chambers can form a single row by making the teeth somewhat narrower than the spaces between the teeth and securing the two piezo-sheets 20, 21 on the support 27 accordingly. Fig. 5 shows another embodiment of an inkjet array. With this type of inkjet heads too, ink droplets are released from small nozzle openings by means of an acoustic pressure rise in an ink chamber situated behind each nozzle opening. The surface tension of the ink prevents ink from emerging spontaneously from the nozzle opening. The pressure rise in the ink chamber is produced by an electrical pulse applied to a piezo-electric element. Since a number of this type of identical elements is used in the head, a large number of droplets can be jetted simultaneously. By movement of a receiving medium at the correct speed a short distance (0.5 - 2 mm) along the head and controlling the piezo-elements image-wise each separately, it is possible to build up an image consisting of a number of ink dots. Fig. 8 shows a single element of the array according to Fig. 5. The piezoelement 43 is provided with electrodes (not shown) with which a sinusoidal or pulsed voltage can be applied across the element 43. As a result the piezo-element 43 oscillates and this oscillation is transmitted to the ink chamber 50 which can thus perform a movement in the direction of arrow 55. The voltage required across the piezo-element is typically 5 to 50 volts. This voltage is dependent on the thickness of the piezo-element, the volume of the piezo-element, the rigidity of the connection between the piezo-element and the ink chamber 50, the dimensions of the ink chamber, and other physical properties of the ink and the droplets. Acoustic pressure waves will be generated in the ink chamber 50 by its acceleration and are propagated in the ink chamber at the speed of sound. The speed of sound in the ink depends, in the present configuration, on the ink properties, the ink volume, and also the compliance of the walls of the ink chamber. A characteristic measurement of the deflection of the ink chamber is 50 - 500 nanometres, and 0.1 - 2 bar for the pressure wave amplitude. By correct coupling of the acoustic impedance of the nozzle 49 and the ink chamber 50 it is possible to bring into motion in the nozzle the liquid which can be regarded as incompressible, speeds of flow better than 10 m/s being possible. The ink emerging from the nozzle 49 is then formed into a droplet by the action of the surface forces. By correctly adjusting the piezo-element control, particularly as regards pulse width, it is possible to generate in the ink chamber pressure waves which by interference yield a high amplitude and thus a high droplet speed for a relatively low control voltage. The movement of the ink holder can be also be used to ensure that there is correct breaking off of the droplet which is ejected. In this way another advantage can be obtained in the final speed of the droplet, and in the prevention of small satellite droplets which have an adverse effect on print quality. The ink is supplied to the ink chamber 50 via a feed duct 45 disposed in a support 40 (of metal or plastic). The ink chamber 50 is applied to support ribs 47 and 48 by a flexible glue connection. The electrical signals are supplied via connecting strip 46. A glass plate 42 is disposed between the ink chamber 50 and the piezo-element 43 to close off the ink chamber 50. In Fig. 5, a number of elements in accordance with Fig. 8 are disposed on a holder 40. The numbering of Fig. 5 is identical to that used in Fig. 8. As shown diagrammatically, the array in Fig. 5 is made up of a set of identical elements each consisting of a finger 43 of piezo-electric material and an elongate ink chamber 50 with a nozzle opening 49 hard-coupled to the piezo-finger. The ink chambers (50) are separated from each other and lie in a first plane. The piezo-finger is provided with electrodes (not shown), by means of which a sinusoidal or pulsed voltage can be applied, so that the piezo-finger can bring the ink chamber into motion and can thus eject a droplet. In comparison with known piezo-electric high-density multinozzle inkjet heads, all the individual adjacent ink chambers in this invention are decoupled and the ink chambers can move entirely independently of one another. The advantage of this is that the ink chamber does not have to be deformed by the piezo-electric actuator in order to generate a pressure rise in the chamber. Another advantage is the improved acoustic insulation between adjacent inkjet elements. The complete decoupling between neighbouring ink chambers in combination with the high density integration of the elements is possible in this invention because use is made of elongate ink chambers and elongate piezo-fingers which extend substantially in continuation of one another and which are designed in a flat configuration. The piezo-actuators and the ink chambers are formed from flat sheets of material. The ink chambers 50 and the nozzle openings are made by anisotropic etching in silicon. High dimensional accuracy can be achieved with this technique. The ink chamber and the nozzle are closed at the top by a Pyrex glass cover 42. The connecting technique used in this connection is anodic bonding. The advantage of this is that no glue has to be used which might clog the ink ducts. The thickness of the silicon layer in which the duct structure is made is typically 200 to 400 microns, the thickness of the glass cover is typically 100 to 200 microns. The depth and the width of the ink chamber itself is typically 75 to 200 microns. The nozzle openings through which the droplets are ejected have a typical dimension of 20 to 50 microns. Good droplet formation results are obtained with ink chambers having a length of some millimetres. The length of the ink chamber determines the natural inherent frequency of the ink column in the chamber. This frequency can couple with natural frequencies of the piezo-electric actuator. More particularly the amplitude of the voltage required can be controlled in this way. Typical liquid natural frequencies in the ink chamber are in the range from 30 to 150 kHz. The ink chambers of the nozzles can naturally also be made by means of other materials and forming techniques. The piezo-elements are sawn from a flat piezo-electric material, to both sides of which the electrode material is applied before sawing. The piezo-fingers 43 have a typical height of 50 to 500 microns, a width of 75 to 400 microns. The length of the piezo-fingers is some millimetres so that each actuator has a rod-like shape (1-20 mm). The electrodes of each individual piezo-finger are electrically connected to the driver IC's (not shown in the drawings). Like the liquid column in ink chamber 50, the piezo-element also have natural frequencies which are important to the good action of the droplet generator. Piezo-element natural frequencies have been measured between 20 kHz and 500 kHz. Voltages required to eject the droplets are typically 5 to 50 volts. In order to avoid cross-talk between the individual piezofingers in the cam structure, the fingers can be separated completely by sawing-out the bridges between these fingers. The silicon/glass ink chambers are connected to the piezo-fingers 43 by means of glues (e.g. Araldite AV 138 containing approximately 30% aluminium oxide), but other connecting techniques are possible. This rigid connection between the ink-chamber (50) and the related piezo-actuator (43) lies outside the first plane. This means that no piezo-material is situated between the separate ink-chambers so that a high density of elements can be achieved. The quality of the connection is very important, because it determines how well the piezo-actuator can transmit the acoustic energy to the ink. At the other end the piezo-elements are glued to a holder. In addition, the ink chambers are supported at another two points by thin strips which stand on the holder. This support can also be constructed in any other manner. To achieve a more rigid construction of the array the spaces around the piezo-actuators (43) and also around the ink-chambers (50) (Fig. 5) are provided with an elastic material. This material can also be used for forming the feed duct (45) whereby leakage of ink round the ink-chambers (50) is effectively prevented. Fig. 6 is a top plan view of the inkjet array according to Fig. 5 and Fig. 7 is a front elevation. These Figures use the same numbering as Figs. 5 and 8. The shape of the nozzle 49 is readily visible from Figs. 6 and 7. It has been found that instead of the rectangular nozzles used here it is possible to use other shapes, such as round or oval, which may or may not be flattened on one side. The inkjet heads described are not only suitable for inks liquid at room temperature, but also hot-melt applications in which the heads are brought to a temperature at which the hot-melt inks are liquid. The holder (40) can be provided with a heating element to bring the whole array to a temperature between 100"C and 1500C whereby hot-melt inks become liquid. Under these circumstances the demands for the adhesive between the piezofingers (43) and the ink-chambers are different in relation to the demands for use at room temperature. A very good result was achieved with a two component epoxy resin comprising in component A. A reaction product of Bisphenol-A and Eplichlorhydrine filled with about 10% by weight Aluminium-silicate. The Bcomponent comprises a mixture of about 50 - 60% by weight pyrmelliticdianhydride and about 40 - 50% mica (Eccoband 104). After mixing of 100 parts by weight of component A and 64 parts by weight of component B at 60"C, hardening takes place at 120"C. The ink chamber array can also be made in some other way, e.g. by disposing a number of glass capillary tubes next to one another, with or without intermediate spacing, and connecting them by a suitable plastic to form a sheet-like tube structure. This is secured to the piezo-sheet in the same way as described with reference to Figs. 5 - 8. By heating and then stretching the tubes constrictions are formed, which act as nozzles.

Claims: CLAIMS 1. An inkjet array for a printer, comprising a holder (40) and a number of elements, each element comprising an elongated ink chamber (50) emptying itself into a nozzle (49) and means for providing it with ink, a piezo-actuator (43) rigidly secured to the ink chamber (50) and the holder (40), the ink chamber being adapted to be brought into motion in a direction (55) substantially in parallel to the axis of the chamber (50) in response to an image signal such that an ink droplet is ejected from the nozzle (49), characterised in that the ink chambers (50) lie in a first plane, separated from each other and can each be brought into motion separately. 2. An inkjet array according to claim 1, characterised in that the rigid connection between each ink chamber (50) and its related piezo-actuator (43) lies outside the first plane. 3. An inkjet array according to claim 1 or 2, characterised in that each piezoactuator (43) is of a rod-like shape. 4. An inkjet array according to one of the previous claims, characterised in that the ink chamber (50) is formed from silicon and closed on one side with glass. 5. An inkjet array to one of the previous claims, characterised in that the space between the chambers (50) and between the piezo-actuators (43) is filled with an elastic material. 6. An inkjet array to one of the previous claims, characterised in that the holder (40) comprises a heating element.

Inventors: ENGELS PETER WILHELMUS HUBERTU, VAN BEEK FREDERIK MARIA, WITTEVEEN BONTKO, ENGELS, PETER, WILHELMUS, HUBERTUS, VAN BEEK, FREDERIK, MARIA, WITTEVEEN, BONTKO
Applicants: BEEK FREDERIK MARIA VAN, ENGELS PETER WILHELMUS HUBERTU, OCE NEDERLAND BV, WITTEVEEN BONTKO, ENGELS, PETER, WILHELMUS, HUBERTUS, OCE-NEDERLAND B.V., VAN BEEK, FREDERIK, MARIA, WITTEVEEN, BONTKO
Application year: 1994
Main classification: B41J2
IPC codes: B41J2